Development of lead free electroless nickel plating systems and metal thin films on silicone and nafion membranes

2.6.2.1 Mechanical properties of EN deposit 362.6.2.2 Internal stresses in EN deposit 362.6.2.3 Electric and magnetic properties of EN deposit 372.6.2.4 Corrosion resistance of EN deposi

DEVELOPMENT OF LEAD-FREE ELECTROLESS

NICKEL PLATING SYSTEMS AND METAL THIN

WANG KE

NATIONAL UNIVERSITY OF SINGAPORE

2008

DEVELOPMENT OF LEAD-FREE ELECTROLESS

NICKEL PLATING SYSTEMS AND METAL THIN

WANG KE

(M.Sc., CUGB; M.Eng., CUGW)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2008

I would like to express my sincere gratitude to my supervisor, A/Prof Hong Liang, and my co-supervisor, Dr Liu Zhao-Lin, for their invaluable guidance and suggestions, continual encouragement, great patience and support throughout the course of my study

I would like to specially thank all the technical and clerical staff in the Department

of Chemical & Biomolecular Engineering for their assistance in the set-up of experimental systems and in the use of materials characterization equipments Thanks are also extended to Mr Yin Xiong, Ms Zhang Xinhui and Ms Tay Siok Wei for their supportive comments and cheerful assistance

I am extremely grateful to my beloved family members for their love and support throughout the course of this program

Finally, I would like to thank to National University of Singapore for granting me a research scholarship throughout this study period

2.1 Electrochemical metal deposition methods 9

2.1.1 Electrolytic metal deposition/plating 9 2.1.2 Electroless metal deposition/plating 11 2.1.3 Advantages of electroless deposition 14

2.5.1.1 Intrinsically active materials 302.5.1.2 Extrinsically catalytic materials 302.5.2 Effects of variables on the ENP process 31

2.6 Structure and properties of electroless nickel (EN) deposits

35

2.6.2.1 Mechanical properties of EN deposit 362.6.2.2 Internal stresses in EN deposit 362.6.2.3 Electric and magnetic properties of EN deposit 372.6.2.4 Corrosion resistance of EN deposits 372.6.2.5 Wear properties of EN deposits 39

2.8.1 Inorganic substitute stabilizers 442.8.2 Organic substitute stabilizers 45

2.9 Metallization of hydrophobic silicone elastomer 48

2.9.1 Metallization of polymers by different methods 482.9.2 Metallization of Poly(dimethylsiloxane) (PDMS) 50

2.10 Development of Pt-based electrocatalysts for proton exchange membrane fuel cells (PEMFCs)

52

Chapter 3 Roles of Sulfur-Containing Amino Acids

in Electroless Nickel Plating Bath

3.2.4 Assessment of ENP bath stability 62 3.2.5 Evaluation of corrosion resistance 623.2.6 In situ adsorption of the two amino acids on fresh

nickel powders

63

3.3.1 An investigation of the dual effects of the two

S-containing amino-acids on ENP rate

64

3.3.2 Nature of S-containing group and the composition

Chapter 4 The Role of Bi3+-Complex Ions as the Stabilizer

in the Electroless Nickel Plating Process

4.3.1 Influence of Bi3+-complex ion on anodic reaction

of hypophosphite

87 4.3.2 The critical role of metal colloidal particles

4.4 Conclusions 105

Chapter 5 Exploring the Phosphine Ligands as Stabilizer

for the ENP System

5.3.1 The state of phophine compounds in ENP aqueous

5.3.4 How will the use of phosphine stabilizer affect the

heat treatment effect?

129

5.3.5 The performance of phosphines in a continuous

Electromagnetic Interference (EMI)

6.2.1 Materials and sample preparation 143

6.2.5 Field emission scanning electron microscopy (FESEM)

145

6.2.8 Measurement of magnetic property 146

6.3.1 Effect of surfactant on the crosslinking of PDMS 147 6.3.2 Wettability of modified PDMS 148

6.3.3 Attaching a TiO2 layer to the PDMS surface via gluing approach

149

6.3.5 Deposition of Pd on modified PDMS 1516.3.6 Surface/cross-sectional morphology and

composition of Ni deposit on modified PDMS

6.4 Conclusions 165

Chapter 7 Electroless Deposition of Anode Catalyst on

Nafion® Membrane – A New Approach for the Fabrication of MEA

7.3.1 Deposition of Pd on Nafion® membrane 173 7.3.2 Electrolessly plated Pt on Nafion® membrane 174 7.3.3 Electrolessly plated Pt/Au on Nafion® membrane 177 7.3.4 Catalytic performance of the fabricated anodes in

8.2.2 Theoretical models for describing working

mechanisms of organic stabilizers

190

Electroless metal deposition/plating, as a versatile surface finishing technology, has been successfully applied in many industries to date Among the established electroless plating systems, the electroless nickel plating (ENP) system is of the most importance due to its significant industrial value ENP generally produces an integrity Ni-P alloy film on the target substrate with very strong adhesion strength and unique physical and chemical properties, such as, hardness, corrosion resistance, wear resistance and so on

Being an autocatalytic reaction at the plating frontier, the ENP process is vulnerable

to decompose at any time due to the presence of insoluble colloidal particles in the plating solution as the plating proceeds In most of commercial ENP applications, lead is introduced in the ppm level as the stabilizer to ensure a stable plating process that consists of several metal-turnover (MTO) rounds But the co-deposition of Pb(0) atom in the EN deposition layer and the presence of Pb2+ ion in the spent solution cause harmful effects to the human beings and high cost for the waste-solution treatment Hence, it has been an urgent issue to explore a less toxic or environmental friendly stabilizer in place of Pb2+ ion in ENP solutions

Another challenge to the development electroless metal plating is how to apply this technique in the arena of Hi-tech, i.e., biomedical and new energy industries, in which the metallization of polymer materials is an essential step

applications because of its excellent thermal and chemical stability The surface metallized PDMS is expected to find applications in the medical/health industry However, electroless metallization on PDMS is seldom reported due to its high hydrophobicity

Proton exchange membrane fuel cell (PEMFC) is regarded as an alternative power generator to the combustion engine for PEMFC uses hydrogen fuel and has a higher efficiency, higher power density and zero pollutant emissions The performance of PEMFCs greatly depends on the membrane electrode assembly (MEA), in which an electrolyte membrane is sandwiched between two electrodes Among different electrolyte membranes, Nafion® is the most commonly used because of its good mechanical and chemical stability and high proton conductivity The enhancement

of Pt utility and improvement of the endurance of Pt catalyst to carbon monoxide and other oxidative intermediates are the key issues which will affect the market acceptance of PEMFC Electroless Pt and Au plating approaches were attempted in this thesis work with the aim to deposit Pt or Pt-Au bimetallic catalyst directly on the surface of Nafion® membrane Here the built Pt-Au bimetallic nanoparticle is anticipated to improve the resistance of the Pt catalyst to poisoning adsorbates

The interest of this PhD research is to investigate solutions to the above problems, namely exploring effective substitutes for lead salt stabilizer in ENP system and advancing electroless plating technology for the fabrication of special devices In the first part of this thesis, three kinds of environment friendly and effective bath stabilizers are presented They are sulfur-containing amino acids (cysteine and

and Bi -complex ions stabilize the ENP bath by controlling the oxidation rate of hypophosphite ions at the plating surface Each of them exhibited a wide effective stabilizing range from 10-9 to 10-5 mol/L before the critical concentration, above which the EN plating becomes idle Below the critical concentration, the concentrations of stabilizers did not show significantly effects on the characteristics

of ENP process and EN deposit Although the sulfur-containing amino acid type and

Bi3+-complex ion type are anodic type stabilizers, they are associated with different stabilization chemical mechanisms

Unlike sulfur-containing amino acids and Bi3+-complex ions, phosphines are a type

of cathodic stabilizer that prevents the ENP process from decomposition through suppressing the rapid reduction of Ni2+ ions at highly reactive Ni surface sites And consequently, there is no a critical concentration for them It is unique for phosphines that the phosphorus-content in the Ni-P deposition layer gradually increases by increasing the concentration of phosphines in the solution, which results in the improvement of anti-corrosion property of as-deposited Ni-P plating layer In addition, thermal annealing of the Ni-P layer could largely promotes its corrosion resistance by almost 20 times due to the formation of diffusion layer between the brass substrate and crystalline layer

In the second part, methods for metallization of PDMS and Nafion® membrane are reported Firstly, a method to electroless plate a well-adhered Ni/P alloy thin film on the surface of modified PDMS was successfully developed At the first, PDMS was modified by including an appropriate surfactant in the PDMS matrix and partially

surface; and then, ENP was carried out on the surface of modified PDMS substrate after activated in a Pd/SnCl2 suspension The deposited Ni-P alloy film is rough and irregular in structure and shows high electrical conductivity and an electromagnetic interference (EMI) shielding property

Secondly, methods to deposit PEMFC anode catalyst directly to Nafion® membrane

by means of electroless plating, in which Pt or Pt-Au bimetallic thin layer consisted

of highly dispersed particles with size smaller than 100 nm, were successfully developed The size of deposited metal particles and the surface morphology of deposition layer were found to strongly depend upon the plating time According to the assessment in a single stack PEMFC, both of Pt and Pt-Au catalysts displayed better electrode reactivity than the normally used carbon black supported PtRu catalyst; whereas Pt/Nafion® anode showed better cell performance than Pt-Au/ Nafion® anode when the current density was above 1.4 A/cm2

Keywords: electroless nickel plating (ENP), stabilizer, sulfur-containing amnio

acids, Bi3+-comples ions, phosphines, stabilization mechanism, surface metallization, poly(dimethylsiloxane) (PDMS), electromagnetic interference (EMI); Nafion®membrane, polymer electrolyte membrane fuel cell (PEMFC), Pt-based catalyst

Symbol Description Unit

C S c critical stabilizer concentration in Eq (4.1) mol/L

R Ni deposition rate of nickel in Eq (4.1) mg/cm2hr

R Ni 0 nickel deposition rate without stabilizer in Eq (4.1) mg/cm 2 hr

R P deposition rate of phosphorus in Eq (4.1) mg/cm 2 hr

R P 0 phosphorus deposition rate without stabilizer in Eq (4.1) mg/cm 2 hr

Fig 2.1 Schematic Representation of Electrolytic Metal Deposition 11Fig 3.1 Influence of concentration of sulfur-containing amino acids on

nickel deposition rate

65

Fig 3.2 Current-potential curves of anodic oxidation of hypophosphite

on nickel at different cysteine concentrations (mol/L) 1) 10-5,

2) 10-6, 3) 10-8, 4) 0, 5) 2×10-4, 6) 10-3

67

Fig 3.3 Peak values of anodic current density vs the corresponding

concentrations of sulfur-containing amino acids

67

Fig 3.4 Schematic graph for interaction between sulfur-containing

amino acids and positively charged phosphorous center at

plating surface

69

Fig 3.5 The UV-Vis spectra of nickel solution with methionine as

stabilizer 1) Methionine 0.05M, 2) Nickel sulfate 0.05M, 3)

Methionine 0.05M + Nickel sulfate 0.05M

Fig 3.11 PDS curves of Ni-P coatings with methionine as stabilizer in

different rounds of MTO 1: 1st MTO, 2: 2nd MTO,

Fig 4.1 Structures of three EDTA-derivatives 88Fig 4.2 Influence of existence of Bi3+-complex ions on nickel

deposition rate

89

Fig 4.3 Current-potential curve of anodic oxidation of hypophosphite

on Ni-P electrode in solution containing 0.24 mol/L hypophosphite and 10-6 mol/L Bi3+-EDTADSS

90

Fig 4.4 Peak values of anodic current density vs the corresponding

concentrations of Bi3+-complex ions

91

Fig 4.5 The XPS spectra of (a) Bi 4f and (b) Ni 2p in four samples

after plated in ENP bath at different Bi3+-complex ion

concentrations for 30 mins

92

Fig 4.6 EDX spectra of samples plated in ENP solution of [Bi3+]=10-3

mol/L for different times: a) 1 min, b) 5 min, c) 10 min, d) 20

min (i.e., starting to count from the moment when the brass

substrate is touched by Ni wire)

94

Fig 4.7 Influence of the Bi3+-complex ion concentration on the

deposition rate of a) nickel and b) phosphorus

97

Fig 4.8 TEM photograph of colloidal particles collected from plating

bath with addition of 10-5 mol/L Bi3+-EDTADSS after a 1 hr

plating process

98

Fig 4.9 Dependence of decomposition volume (Vd) on concentration

Fig 4.10 Effect of MTO on nickel deposition rate with using

Bi3+-complex ions as the stabilizer

101

Fig 4.11 PDS curves of Ni-P coatings with Bi3+-EGTADSS complex as

stabilizer in different rounds of MTO

103

Fig 4.12 Corrosion current density vs MTO for three bismuth

complexes

104

Fig 4.13 Surface morphology of the 4 MTO samples by using Bi3+

-(EDTA-OH) complex as the stabilize

105

Fig 5.1 Molecular structures of the three phosphines 114

Fig 5.3 Influence of concentration of phosphines on Ni-P deposition

rate

118

Fig 5.4 The co-deposition of TPP or BDPPM insoluble particles on

the plating surface at high concentraton (10-3 mol/L) 118Fig 5.5 Peak values of anodic current density vs the corresponding

concentrations of phosphines (For TPP and BDPPM the tested

concentration is up to 10-5 mol/L, because neither of them can

be completely dissolved under the experiment conditions

above this concentration)

120

Fig 5.6 Schematic presentation of adsorption of TPP and TPPTS on

Fig 5.7 XPS spectra of P 2P in four samples together with curve fitting 123Fig 5.8 Dependence of decomposition volume on added concentration

of stabilizers

124

Fig 5.9 Nyquist plots obtained for as-plated EN deposits plated from

ENP bath using TPPTS as the stabilizer in 3.5% NaCl solution

128

Fig 5.10 Equivalent circuit for EIS measurements on corrosion

resistance of as-deposited Ni-P deposits plated from ENP bath

using TPPTS as the stabilizer

129

Fig 5.11 X-ray diffraction pattern of ENP deposition player before and

after heat treatment

131

Fig 5.12 Surface morphology of ENP deposit layer generated from the

bath containing TPPTS of 10-5M before and after annealing

treatment

132

Fig 5.13 Nyquist plots obtained for heat-treated EN deposits in 3.5%

Fig 5.14 Equivalent circuit for EIS measurements on corrosion

resistance of heat-treated Ni-P deposits

133

Fig 5.15 Element Analysis on the cross section for EN deposits before

and after heat treatment

135

Fig 5.16 Effect of MTO on the deposition rate with using phosphines

Fig 6.1 Contact angles of water on PDMS before and after

Fig 6.2 XPS analysis of the three PDMS surfaces where TiO2

nanoparticles were embedded

150

Fig 6.3 The appearances of pure and modified PDMS (Before

modification PDMS was colorless and transparent and became

milky after modification.)

151

Fig 6.4 XPS spectra of Pd and Sn on the PDMS surfaces of the three

samples (distinguished by different surfactants used) after they

were activated in acidic Pd-Sn colloidal dispersion

153

Fig 6.5 Surface morphologies of the pure PDMS, the modified PDMS

Fig 6.6 AFM images of pure, modified and Ni plated PDMS 156Fig 6.7 a) Cross-sectional image of plated modified PDMS; b) The

schematic of the cross section of the ENP plated modified

PDMS film

157

Fig 6.8 Elemental depth profiles of the ENP plated PDMS film 158Fig 6.9 Microscopic views of the Ni plated PDMS surface before and

Fig 6.10 a) Schematic presentation of experimental setup for surface

resistivity measurement; b) The graph of I against V

161

Fig 6.12 Schematic presentation of experimental setup for evaluation of

electromagnetic shielding property of the Ni-P layer on the

Fig 7.5 Surface morphology of electroless Au deposits from samples

with different plating time and XPS spectrum of electrolessly

plated Au4f (using sample plated for 30s as an example)

177

Fig 7.6 XPS spectra of PtAu alloy for different Au plating time

(L Pt=0.2 mg/cm2 for all samples)

180

Fig 7.7 Surface morphology of electroless PtAu deposits from samples

with different L Pt (plating time for Au: 15s)

181

Fig 7.8 XRD patterns of a) Nafion® substrate, b) electroless Pt plated

Nafion® substrate and c) electroless PtAu plated Nafion®

substrate (L Pt=0.2 mg/cm2 in samples b and c; Au plating time

=15s in sample c)

182

Fig 7.9 I-V & power density curves for three anode catalysts in

PEMFC, in which PH 2=1 bar, PO 2=1 bar, L Pt=0.2 mg/cm2 and

Tcell=25o

C

184

Table 2.1 Reducing agents for ENP 17Table 3.1 Composition of acidic hypophosphite plating bath 60Table 3.2 Composition of Ni-P deposition layer based on EDX

Table 5.3 Fitted results of Nyquist plots for EN deposits on equivalent

circuit described in Figure 5.14

134

Table 5.4 Ni & P contents in the Ni-P deposits from 4 MTO 137Table 6.1 Composition of Acidic Hypophosphite Plating Bath 144Table 6.2 Surfactants Used for the Preparation of Modified PDMS

Films

147

ENP is a chemical reduction-oxidation process in which the driving force for the reduction of Ni2+ ions is supplied from a reducing agent in solution This process is autocatalytic in nature because the redox reaction is catalyzed by the nickel being deposited itself (Sadeghi, 1983; Schlesinger, 2000) This autocatalytic nature leads

to the inherent instability of ENP bath With the proceeding of plating, colloidal particles of insoluble nickel phosphite can act as highly efficient catalytic sites, and trigger overwhelming deposition of nickel black, known as a “self-accelerating chain reaction” or the plating-out process, and result in failure of plating bath (Mallory, 1990; Riedel, 1991; Schlesinger, 2000) Traditionally Pb2+ ion is employed in ppm

as stabilizer in commercial ENP baths to prolong their service life which not only

improve the overall efficiency of the ENP, but also minimize the waste solution; however, the use of Pb has been restricted worldwide due to its harmful effects to human being through chronic contact (Castellino, 1995) Development of lead-free ENP baths has consequently become obligatory (Chen, 2006) Therefore, it is necessary to explore some non-toxic chemicals to replace lead as the stabilizer in ENP solution and understand the stabilizing mechanisms of these substitutes in ENP process

Polydimethylsiloxane (PDMS) is a very useful material that has broaden applications in automotive, electric and electronics industries, food packaging, composite membranes and medical/biomedical devices (Jershow, 2002; Iwasaki, 2007; Qi, 2007) Metallization of polymer substrate has major applications in the microelectronics industry such as magnetic storage devices and printed circuit boards (Yan, 2004; Azzaroni, 2006) In addition suitably metallized polymers have potential applications in medical/biomedical industry (Metz, 2001; Gray, 2005) Different methods have been developed to metallize many polymers, for example, physical vapor deposition (PVD), chemical vapor deposition (CVD), and electroless plating (Mittal, 1998 & 2001; Kim, 2001; Carlo, 2002; Sipaut; 2007) However, reports have seldom been published for metallizing PDMS, especially for ENP, likely because of its highly hydrophobic nature Consequently it is essential to develop a simple and inexpensive method to modify PDMS surface to make it suitable for metallization by ENP

Nowadays, proton exchange membrane fuel cell (PEMFC) has been expected to be

an alternative power source for automobiles and portable applications due to

cleanliness, security and sustainability (Alkire, 1997; Wei, 2007) Among different available membranes used in PEMFCs, Nafion® and its derivatives have received considerable attention due to their strong chemical resistance and high proton conductivity (Kundu, 2007) Besides the membrane, another key material for PEMFCs is the catalyst In general, platinum and its alloy are employed as the catalyst in the electrodes of PEMFCs, due to their excellent catalytic activity and stability under fuel cell operation conditions (Koh, 2007; Liu, 2007; Mani, 2008; Ramaswamy, 2008) It is well known that the performance of PEMFCs greatly depends on the membrane electrode assembly (MEA) There are two approaches for fabrication of MEAs: (1) catalyst powder or powder type and (2) in situ catalyst formation on the surface of the gas diffusion layer (GDL) or the membrane And generally, the latter method has higher Pt utilization than the former The in situ formation of catalyst primarily includes impregnation and deposition methods Among them, deposition is very attractive due to the formation of high-purity Pt and ease of control catalyst loading (Ayyadurai, 2007) Physical vapor deposition (PVD), chemical vapor deposition (CVD), sputter deposition and electrodeposition have been reported to deposit catalyst on the surface of GDL or the membrane (Debe,

2000 & 2003; Hampden-Smith, 2002; Wu, 2007) Electroless platinum plating (EPP), however, a much cheaper technique, is still seldom employed for the in situ

Pt deposition Therefore it is reasonable to develop a process for the fabrication of MEA with EPP

1.2 Objectives of this thesis work

The two main purposes of this thesis work are (i) replacement of lead by other environment-friendly organic and inorganic compounds as the stabilizer in acidic ENP solutions and investigation of the reaction mechanisms of these substitutes at the plating surface during ENP process, and (ii) developing electroless plating methods to modify the surface of polymers and consequently broaden the potential applications of these modified polymers in biomedical and new energy industries The details of research execution are highlighted as follows:

1 Identifications of three groups of environmental-friendly stabilizers those are sulfur-containing amino-acids, Bi3+-complex ion and phosphines, to replace Pb in the ENP solution, and proposal of suitable ligands to introduce indissoluble Bi3+ion and phosphines into the weak acidic ENP solution

2 Mechanism study of the stabilizing effect of each substitute stabilizers in ENP process, which includes (i) exploring the reactions between the stabilizers and the deposited Ni0 atom at the plating surface and the Ni2+ ion in the bulk solution, (ii) investigating the effect of stabilizers on the anodic oxidation of hypophosphite at the plating surface, the rate determining step of ENP process, (iii) comparing the stabilizing capability of different stabilizers, and (iv) simulating the relationship between stabilizer concentration and nickel deposition rate for Bi3+-complex ion

3 Investigation of the effects of substitute stabilizers on ENP process and EN deposit, such as plating rate, deposit composition, surface morphology et al and their performances in long-time ENP operation

4 Development of a reverse surface roughness approach to surface metallize the hydrophobic PDMS substrate by ENP and investigation of the unique properties

of Ni-P deposit on PDMS to highlight its potential application in biomedical industry

5 Development of an electroless plating method to directly deposit Pt or Pt-Au bimetallic thin layer on the surface of Nafion® membrane as the catalyst for PEMFC and characterization of its cell performance

1.3 Thesis organization

Chapter 2 gives a detailed review about the general knowledge of electrochemical metal deposition methods, the basic components of ENP solution and their role in ENP process, detailed process of ENP, the reaction mechanisms having been proposed to explain the phenomena in ENP process and the limitations of them, physical and chemical properties of the EN deposit and its broad applications, substitute stabilizers for lead investigated to date and their influences on ENP process and EN deposit, methods developed for surface metallization of hydrophobic polymers and approaches for fabrication of MEAs for PEMFC

In Chapter 3 the topic of interest is the roles of two sulfur-containing amino acids, cysteine and methionine, as the stabilizer in ENP bath The relationship between nickel deposition rate and concentration of two amino acids was set up and the critical concentrations for cysteine and methionine were determined as well Two stabilizing mechanisms were come up with to explain the different experimental phenomena observed for cysteine and methione in ENP process The influences of these two amino acids on the EN deposits such as P-content and surface morphology were investigated The bath stabilizing capability of cysteine and methionine was assessed through a continuous operation comprising 4 metal-turn-over (MTO) runs and the properties of EN deposits from each MTO run were evaluated

Chapter 4 presents the role of Bi3+-complex ion as the stabilizer in ENP process Asymmetric derivatives of EDTA (ethylene diamine tetraacetic acid) were introduced into the ENP solution as a unique type of ligand to form soluble Bi3+-complex in the weak acidic environment The relation of ENP deposition rate with the stabilizer concentration was investigated and a modified kinetic model was

established which successfully simulate the effect of the stabilizer concentration (C s)

on the deposition rates of nickel (R Ni ) and the phosphorus (R P) respectively on the basis of the electric double layer theory A series of experiments were carried out to clarify how the coordinated Bi3+ ion participates the ENP process and the result shows that Bi3+ ion stabilizes ENP bath through the disproportionational reaction with Ni atom at plating surface which controls the oxidation of hypophosphite In addition, the effects of Bi3+-complex ion concentration on the performance of ENP process and properties of EN deposit were studied in detail as well

In Chapter 5 the effects of phosphine ligands as the stabilizer on the formation and characteristics of EN deposit were investigated Here the hydrophobic phosphines were introduced into ENP solution by firstly forming complexes with high-concentrated lactic acid It was noteworthy that phosphine compounds did not significantly retard the anodic oxidation of hypophosphite at the plating surface as other stabilizers did, because they are a specially cathodic-type stabilizer The P-content of the EN deposit increased with the increasing concentration of phosphines

in the ENP solution which improved of corrosion resistance of EN deposit After annealing at 500 oC for 1 hr, the corrosion resistance of EN deposit was greatly enhanced due to the formation of a three-layer structure Finally the stabilizing capability of phosphines was evaluated by a continuous 4-MTO test and stabilizing mechanisms were established for each phosphine

In Chapter 6 a success method to electroless plate a well-adhered Ni/P alloy thin film on the surface of modified PDMS was developed This methods involved hydrophilization of hydrophobic PDMS, coating TiO2 nanoparticles on the surface

of hydrophilic PDMS, activation of modified PDMS surface and ENP The surface metalized PDMS substrate showed electromagnetic shielding property which is attractive in biomedical industry There were two novel developments in this Chapter: firstly, the development of a reverse surface roughness approach for modification of hydrophobic PDMS which increased the wettability of the PDMS surface and created a half-exposed TiO2 layer at the top of PDMS substrate, this measure was essential for the following ENP; secondly, the development of an irregular and rough Ni-P thin film on the modified PDMS surface by ENP, this

alloyed metal layer brought unique electromagnetic shielding (EMI) property to PDMS substrate Compare with other polymer surface metallization methods, the great advantages of this method come from its low operation cost and simple equipment requirement

Chapter 7 reports an electroless plating method to directly deposit Pt or Pt-Au bimetallic thin layer on the surface of Nafion substrate as the catalyst for PEMFC Firstly, a stable electroless Pt or Au plating system containing several stabilizers was developed, in which Pt or Au could be electroless deposited on activated Nafion®substrate at a reasonable plating rate The relation of particle size and distribution with the metal loading or plating time was studied and the optimum plating conditions were determined Finally, the cell performance was tested in which electroless deposited Pt or Pt-Au Nafion membrane was used as the anode for reduction of hydrogen, and both of them showed a higher output power than the traditional PtRu anode

Finally, conclusions of this thesis and recommendations for future work are given in Chapter 8

Chapter 2 Literature Review

2.1 Electrochemical metal deposition methods

In general, the electrochemical metal deposition methods can be classified as electrolytic metal deposition/plating and electroless metal deposition/plating, in which metals are deposited from a liquid medium, generally aqueous, by electrolysis

or electroless (autocatalytic) means The former may be defined as a process in which the item to be coated with metal is made the cathode in an electrical circuit The electroless process, as its name suggests, uses no external current source, in which the reduction of metal ions to the metal atoms is due to the presence of chemical reducing agents in solution (Gawrilov, 1974; Riedel, 1991; Paunovic, 2006)

It is well known that the electrochemical metal deposition that will be discussed in detail in the following parts has numerous industrial applications The study on the electrolytic and electroless metal plating is, hence, of great significance to current research in metal deposition

2.1.1 Electrolytic metal deposition/plating

Electrolytic metal deposition/plating is defined as a process in which the item to be coated with metal is made the cathode in an electrical circuit In this process, the

electron that derives from the dissolving metal (the same element as the deposited metal) worked as the anode plays an integral role in the reaction

For electrolytic plating, the electrochemical nature of the metal deposition process can be just simplified written as a chemical reduction-oxidation reaction as the Equation 2.1 (Raub, 1980; Riedel, 1991):

lattice Oxidation

duction Z

Me + + −←⎯Re⎯⎯/⎯ ⎯→

(2.1)

Where Me is metal atom; z is valence of the metal ion and e−is electron

Typically, an aqueous metal solution is used as the electrolyte because of their excellently electrical conductivity and the high solubility of most metallic salts in water Other advantages include its low cost and operation safety (Raub, 1980; Riedel, 1991) The schematic drawing of electrolytic metal plating is shown in Figure 2.1, in which the cathode and the anode are connected via an external electrical source, to produce an electric cycle (Raub, 1980; Riedel, 1991)

In the case of electrolytic metal deposition from an aqueous solution, the metal is generally not present as a simple ion, but rather as a hydrated complex It is believed that the hydrated metal ions are discharged when moving from bulk solution to the substrate surface As the hydrated ion comes close to the diffusion layer, the surrounding water molecules have to re-orientate themselves due to the electrostatic expelling force from the substrate surface Finally, at the inner Helmholtz layer, the metal ion becomes naked due to the very high electrostatic potential Hence it is the

pure metal ion, instead of hydrated ion, that is involved in the actual electrochemical process at the cathode surface (Gerischer, 1957; Riedel, 1991) The reduced atom is initially adsorbed on the cathode surface, then migrates along the surface to a growth point and is incorporated into the metal lattice (Raub, 1980; Riedel, 1991)

M e t a l

Anode

S u b s t r a t

e Cathode

Me z+

Figure 2.1 Schematic Representation of Electrolytic Metal Deposition

2.1.2 Electroless metal deposition/plating

Electroless metal plating is a chemical deposition process, in which the deposition of

a metal from its electrolyte solution relies on chemical reduction rather than external electrical source to supply the necessary electrons In other words, the chemical plating process involves the transfer of electrons between reacting chemical species

All chemical deposition processes can be classified into following two groups according to their reaction mechanisms (Riedel, 1991):

Š Deposition by ion exchange or charge exchange (immersion deposition or displacement reaction):

When a metal substrate is immersed in a metal salt solution of a second (deposition) metal, the simultaneously spontaneous oxidation of the substrate and deposition of the second metal occur at the surface of metal substrate Here the substrate atoms loss electrons, become cations and diffuse into the solution The deposition metal ions accept electrons from the substrate and deposit on it The reaction equations for this process can be expressed as:

Š Deposition of metal from a solution containing reducing agents (electroless plating):

This process is a continuous metal deposition and a thick deposit is often obtained Such deposition occurs only at certain catalytically active surface, which means the deposition reaction initially occurs at the surface of catalytic metal substrate exclusively and subsequently continues on the initial deposit In this case electrons first transfer from the reducing agent to the surface of metal substrate, and then from the surface to the deposition metal ions The reaction equations for this process can

2.1.3 Advantages of electroless deposition

Electrolytic deposition process is technically more straightforward and less expansive than electroless deposition However, electroless deposition provides the following advantages over electrolytic deposition:

♦ More uniform and less porous deposits are produced on complex parts

♦ Powder supplies and electrical contacts are unnecessary

♦ Deposits can be produced directly on non-conductor

♦ Deposits have unique chemical, mechanical or magnetic properties

For above reasons, electroless deposition of metals has been developed to be a surface coating technology of great importance The metals those can be deposited

by electroless plating include nickel, copper, platinum, gold, silver and cobalt which are autocatalytic in nature Among them, electroless nickel plating (ENP) is undoubtedly the most important one in use today The widespread applications of ENP are contributed to its uniquely physical and chemical properties (Mallory, 1990)

2.2 History overview of ENP

As early as 1844, Wurtz discovered that metallic nickel could be deposited from its aqueous salt solution by reduction with hypophosphite However, Wurtz only obtained a black power It was Breteau who first obtained the bright metallic deposits of nickel-phosphorus alloys in 1911, almost 70 years after Wurtz’s

discovery The first patent on an ENP bath was issued to Roux in 1916 These baths, nevertheless, decomposed spontaneously and formed deposits on any surface that was in contact with the solution (Mallory, 1990; Riedel, 1991)

Interest in ENP was really triggered off after Brenner and Riddell (1946) developed

on of ENP

he uniquely physical and chemical properties of an ENP deposit depend on its

a practical ENP system for achieving continuous ENP The technology used in their ENP system has been the main scientific basis for that used today The first commercially available ENP solution was produced by General American Transportation Cooperation (G.A.T.C.) under the trade name “Kanigen” in 1955 Apart from “KANIGEN” process, in the mid end of 1970’s, the “Durnicoat” process based on sodium hypophosphite reduction and “Nibodur” process using sodium borohydride as the reducing agent were developed (Mallory, 1990; Riedel, 1991) From then on, the ENP process has been investigated further and expanded by many workers to its present state of development

2.3 Basic compositi

T

composition that, in turn, mainly depends on the formula of the ENP bath It is noteworthy that the ENP bath has undergone modifications without changing the basic components since the inception of this technology The essential components for an ENP bath are a nickel source, a reducing agent, suitable complexing agents, stabilizers and surfactants

2.3.1 Nickel source

he nickel source in ENP solution can be provided by nickel sulfate, nickel chloride,

or most industrial applications, the nickel concentration of an ENP solution is

T

or nickel acetate (Chen, 1997) Among them nickel sulfate is preferred, especially for acid ENP solutions Nickel chloride was used as the nickel source in the early stage of ENP development But the chloride anion not only brought tensile stress in ENP deposit, but also brought a deleterious effect on corrosion resistance when the ENP bath is used to plate aluminum, or when the plated ENP film was used to work

as a protective coating in corrosion applications (Mallory, 1990; Jiang, 2000) Consequently, nickel chloride is only used in very limited applications today (Delaunois, 2001; Haque, 2005; Zhong, 2006) When use nickel acetate as the nickel source, there is not significant difference in bath performance or deposit quality compared with nickel sulfate Because nickel acetate is more expensive than nickel sulfate, it is seldom used in industrial sectors either Nickel hypophosphite is an ideal nickel source By using it the accumulation of sulfate anions will be eliminated and the buildup of alkali metal ions will be kept to minimum when the ENP bath is

in long-time operation Unfortunately these advantages gained by nickel hypophosphite are offset by its high price (Mallory, 1990; Jiang, 2000)

F

about 6.5 ± 1.0g (0.09-0.13 mol/L) The nickel concentration shows no or little influence on the plating rate, if it is equal to or greater than approximately 5g/L (0.085mol/L) In another word, the plating reaction is zero order with respect to the nickel ion concentration (Baldwin, 1968)

2.3.2 Reducing agents

part from the nickel source, the most important bath component is the reducing

Table 2.1 Reducing agents for ENP (Mallory, 1990; Jiang, 2000)

Redu

electrons ENP solution

ox Potential (V)

A

agent The common used reducing agents, in order of popularity, are sodium hypophosphite, sodium borohydride, dimethylamine borane (DMAB) and hydrazine, which are structurally similar in that each contains two or more reactive hydrogens (Taheri, 2001) In each case hydrogen is evolved during the ENP process and the nickel reduction is said to result from the catalytic dehydrogenation of the reducing agent Table 2.1 summaries the four reducing agents with some of their properties

cing agent Formula Free pH range of Red

Unlike the electrolytic nickel plating, the EN deposit is not pure nickel but alloy

mong these different ENP processes, the Ni-P based alloy process is predominant,

0 2

3 2 2

2 2

Equation 2.6 completely fails to account for the phosphorus component of Ni-P

the reactants The

co-the concentration of hypophosphite on co-the

He pointed out that the increase of

agents, sodium borohydride is the most widely used because

(2.7)

alloy and relationship between the nickel deposition and

deposition of phosphorus involves a secondary reaction that will be discussed in the following In addition, Gutzeit (1959) has shown that the plating rate is dependent

on hypophosphite concentration, but independent on the nickel concentration when

it is above 0.02 mol/L

Lee (1963) reported the influence of

phosphorus-content (P-content) of Ni-P alloy

hypophophite concentration in the ENP solution resulted in increasing P-content in the Ni-P deposit The P-content, typically 7-13 wt.%, in the deposit determines crystal structure of deposit, which in turn influences its properties (Ma, 1986; Tulsi, 1986) Low phosphorus EN deposits (P-content < 7% wt) are microcrystalline, that

is, they consist of many very small grains Deposits have higher P-content can be considered amorphous (Yamasaki, 1981)

Borohydride is used to reduce nickel ion in basic ENP solution Among the borohydride reducing

of its availability (Singh, 1989; Mallory, 1990) The borohydride ion is a powerful reducing agent, the decomposition of which yields eight electrons for the reduction reactions in ENP process Generally the electroless deposition of nickel using borohydride as the reducing agent can be written as follows:

0 2

4 2